In Long-Term Evolution (LTE), in uplink, when the total power required by different physical uplink channels is beyond the User Equipment (UE) maximum transmission power, power scaling is needed. The rules about how to do power scaling across different physical uplink channels or even across different nodes can be summarized as below.
For Carrier Aggregation (CA), due to multiple Physical Uplink Shared CHannel (PUSCH) transmissions and simultaneous PUSCH and Physical Uplink Control CHannel (PUCCH) transmissions, the available transmit power may be insufficient for a UE. When the total required transmission power exceeds the maximum allowed UE transmission power, UL power scaling shall be applied to reduce the total transmission power to fall below the maximum allowed UE transmission power. Currently in LTE, the power scaling priority rules are as following when there are multiple physical uplink channels:                when there are PUCCH and PUSCH with Uplink Control Information (UCI), the PUSCH shall be scaled firstly;        when there are PUSCH with UCI and PUSCH without UCI, the latter shall be scaled firstly;        when there are multiple PUSCHs without UCI, these PUSCHs are scaled equally (i.e. with equal priority).        
For dual connectivity, two different designs have been done, for uplink power control design, based on what level of synchronization the UE can assume between the Master evolved NodeB (MeNB) and Secondary eNB (SeNB), according to the above aspects. This may be translated into two Power Control Modes (PCMs), i.e., PCM1 that can handle synchronization level up to mode 1 and PCM2 that can handle synchronization level up to mode 2.
DC power control mode 1 (PCM1) refers to the following power control behavior:                The power is distributed among PUSCH/PUCCH and across cell groups (CG) in the same manner as for CA within Release 11. With the exception that if the same UCI type collides, the transmission power of physical uplink channels of Secondary Cell Group (SCG) shall be scaled first before scaling the transmission power for physical uplink channels of Master Cell Group (MCG).        
DC power control mode 2 (PCM2) refers to the following power control behavior:                Since there can be a timing difference between two connectivity's, power scaling shall be performed for the physical uplink channels of the connectivity with later transmission timing over the air.        
For the 5th Generation (5G) network or the New Radio (NR), it is supposed to support multiple types of services using common Radio Access Network (RAN), including, but not limited to, enhanced Mobile BroadBand (eMBB), massive Machine Type Communication (mMTC) and Ultra-Reliable and Low Latency Communication (URLLC). These services require different Quality-of-Services (QoS), e.g., delay, data rate and packet loss rate.
To be specific, URLLC is a category of services defined in 3GPP TR 22.862, Version 14.1.0. For URLLC services, both high reliability and low latency are required. But, URLLC usually has very low data rate and possibly sparse data transmission. For mMTC services, long battery lifetime is typically required, but low delay or high data rate is not required, often combined with small infrequent packets. For eMBB services, high data rate is required, and delay can be strict but typically less strict than that in URLLC services.
For URLLC services, the typical view is that one could not rely much on the Automatic Repeat request (ARQ) protocol to enhance the data transmission reliability and the residual Hybrid Automatic Repeat reQuest (HARQ) BLock Error Rate (BLER) shall be low in order to meet the QoS requirement.
The problem with LTE like power sharing solution is that it does not care about differences among upper layer services. Each PUSCH channel always has a lower priority than each PUCCH channel. And, PUSCH channels carrying pure user data (no UCI) are always treated equally. This is not a problem in LTE as the service provided by LTE is Mobile BroadBand (MBB). However, in NR, a UE can support different slices concurrently, e.g., URLLC and eMBB or mMTC. When the UE is power limited, equal power scaling on URLLC and eMBB is not a good choice as URLLC definitely has higher QoS requirements than eMBB.